If you're worried about an element touching the wort directly, use a HERM coil. Going with a double-jacketed system for mashing seems like a ton of expense for something that probably doesn't warrant that much complexity.

jmo, of course.

EDIT. Sorry. I misread. You want this for the boil. I still think double-jacketed for heating is overkill.

I just finished my electric HERM build. The HLT element just heats water. The boiling kettle is also electric but I've had no detectable problems with the wort contacting the element. I love it - brew in my basement year round. Super stable mash temps.